PHNOM PENH — Cambodia's ailing former king Norodom Sihanouk left his country again for Beijing on Thursday to undergo medical tests, despite a vow to stay in his homeland forever.

The 89-year-old ex-monarch, who is a revered figure in Cambodia, smiled and waved to well-wishers as he was given a red-carpet sendoff by Prime Minister Hun Sen and other senior government officials at Phnom Penh airport.

It is unclear how long Sihanouk, who is travelling with his wife and their son King Norodom Sihamoni, will remain abroad.

The ex-monarch has suffered from a number of ailments in recent years, including cancer, diabetes and hypertension. He has been a frequent visitor to China, where he received the bulk of his treatment.

After returning in October from a previous stay of almost three months in Beijing, Sihanouk told his compatriots that he would never leave Cambodia again, in a speech to mark the 20th anniversary of his return from exile.

But his medical needs have made it impossible to keep that promise, Sihanouk's personal secretary Prince Sisowath Thomico told AFP.

"The Chinese doctors can travel to treat him in Cambodia, but they can't bring their equipment with them so they asked him to go to China for a check-up," Thomico said.

"His health is still good," he added. "He is still strong."

One of Asia's longest-serving monarchs, the former king abruptly quit the throne in October 2004 in favour of his son, citing old age and health problems.

Despite abdicating, the ex-monarch sometimes uses his website to communicate with the outside world.

In his most recent message in early January, he said he wanted to be cremated upon his death and have his ashes kept in an urn inside the Royal Palace, reversing an earlier wish to be buried.